The invention relates generally to the field of digital color image processing and, more particularly, for compensating the image color differences for the applied luminance tone scale.
There exist in the prior art several methods of applying a tone scale function to a color image. The first method involves applying the tone scale function to each of the color signals of the original image. This may be implemented by applying a tone scale function Look-Up-Table to each of the red, green, and blue channels of an image independently. In general, tone scale application by this method has a color characteristic in addition to the modification of the overall image contrast. The image processed by a tone scale function in the manner described will effect each pixel""s hue and saturation.
A second method of tone scale application involves rotating a color image into a space consisting of a luminance signal and several chrominance channels. The tone scale function is then applied only to the luminance channel. The processed image is obtained by applying an inverse matrix to produce transformed red, green, and blue pixel values. This method preserves the original chrominance signal at each pixel. However, because the luminance contrast was modified and the chrominance contrast was preserved, the color saturation of the resulting image may appear artificial and unnatural.
In an effort to address this problem, Viggiano and Wang proposed (1992 TAGA Proceedings: A Comparison of Algorithms for Mapping Color Between Media of Differing Luminance Ranges, pp. 959-974) a method of scaling the chroma signal of a digital color image by a factor midway between unity and the ratio of the L* range of the reproduction and the original. However, this solution does not anticipate tone scale functions that vary the level of compression dependent upon the intensity of the input. Thus, a single factor that scales the chroma signal of the input image is inappropriate for a wide range of possible tone scale functions.
In addition, in U.S. Pat. No. 5,446,504, Wada describes a method to correct color saturation when compressing the dynamic range of an image. However, this method once again determines the correction factor based upon the relationship of the input image dynamic range and the output image dynamic range.
In addition, in U.S. Pat. No. 5,638,138, Hickman describes a method of modifying the luminance signal of an input image, then xe2x80x9cmultiplying the color components by a transfer ratio of modified luminance to unmodified luminance to obtain modified color components.xe2x80x9d However, this method again does not consider that a tone scale function may consist of several ranges of varying level of contrast compression or enhancement.
Thus, none of the methods described in the prior art have determined a specific color adjustment factor based upon both the intensity values of the pixel being processed and the given tone scale function. Consequently, there exists a need for a method of adjusting the color saturation of each image pixel by a factor related to the local slope of the given tone scale function.
The object of the present invention is directed to providing a superior method of compensating the color saturation characteristics of a digital color image for the application of a tone scale function. Briefly summarized, according to one aspect of the present invention, the invention resides in the steps of: (a) receiving a tone scale function; (b) calculating a local slope of the tone scale function for each pixel of the digital color image; (c) calculating a color saturation signal from the digital color image, and (d) adjusting the color saturation signal of the digital color image for each of the pixels based on the calculated local slope.
These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.